1. Field of the Invention
This invention relates to a cathode active material having superior cell characteristics in rechargeable electrochemical cells, more particularly to a carbon-containing composite cathode having a general formula of LiFe1−xMxPO4—C, within 0≦x<1, M is selected from the group consisting of Co, Ni, V, Cr, Mn and a mixture thereof.
2. Description of the Prior Art
Lithium-ton batteries have been a main power source for portable electronic devices, such as cellular phone, laptop, camera and camcorder because of their high energy, high power and long cycling life. These batteries currently use a lithiated transition metal oxide, such as LiCoO2, LiNi1−xCoxO2, or LiMn2O4, as the cathode active material. Such materials contain toxic transition metal and also are high cost; moreover, they are unsafe at the fully charged state. Therefore, extensive researches have recently been focused on searching for the alternative cathode active material that is safe and environmental benign.
U.S. Pat. Nos. 5,910,382, 6,391,493 and 6,514,640 proposed lithium iron phosphate (LiFePO4) as a candidate for the cathode active material of rechargeable lithium and lithium-ion batteries. This material does not contain toxic heavy metal element and starting materials for its synthesis are low cost, compared with the current cathode materials of LiCoO2, LiNi1−xCoxO2, and LiMn2O4, while provides highly thermal stability. Furthermore, the LiFePO4 has a theoretical capacity of 170 mAh/g and a flat discharge potential of 3.4 V versus Li/Li+. These properties make it an attractive candidate for the cathode material of rechargeable batteries. However, LiFePO4 has very low electronic conductivity, the cell using it suffers high electric polarization and poor rate capability. To overcome these problems, a carbon-coating method has been proposed to enhance the electronic conductivity of the LiFePO4 cathode active material. Meanwhile, this method offers an additional advantage. That is, the carbon at high temperature becomes highly reductive, which consequently provides a reductive environment to protect Fe(II) in the LiFePO4 cathode from being oxidized by oxygen in air.
U.S. Pat. No. 6,632,566 discloses that the preparation of LiFePO4 should be conducted under an inert atmosphere having high purity such as N2 and Ar, or under a reductive atmosphere such as a H2/N2 mixture and NH3. Otherwise, Fe (II) in the product will be oxidized to Fe (III) by a trace amount of oxygen contained in the inert gas, which reduces capacity of the LiFePO4. On the other hand, LiFePO4 is an inherently poor electronic conductor. CA Patent 2,270,771 teaches that mixing a small amount of polymer into the starting materials can significantly enhance electronic conductivity of the LiFePO4 as the polymer thermally discomposes to form an electronically conductive carbon that subsequently coats onto the surface of LiFePO4.
Following CA Patent 2,270,771, Huang et al (Solid-State Lett. 4: A170-A172, 2001) heated a composite of the phosphate and a carbon xerogel formed from a resorcinol-formaldehyde precursor to prepare a LiFePO4—C composite, in which the composite cathode achieved 90% theoretical capacity at C/2. However, the procedure of making the composite cathode is rather complicated and very costly, whereas the composite contains about 15% (by weight) of carbon, which reduces the overall specific capacity of the composite cathode since carbon itself does not participate in the cell reactions. To alleviate these concerns, Chen et al (J. Electrochem. Soc. 149: A1184-A1189, 2002) heated a mixture of the starting materials and inexpensive sugar to get a LiFePO4—C composite having reduced particle size and small content of carbon, while providing comparable rate capability. However, the composite cathode such-made suffers a dramatic decrease in the tap density, which hence reduces volumetric energy density of the battery.
WO2004001881 discloses a solution method for preparing LiFePO4—C composite cathode. This method includes dissolving the starting materials in a 1:1 (molar ratio) aqueous solution of polycarboxylic acid and polyhydric alcohol. After polyesterification of the acid and alcohol, the solution is evaporated and then heated in an inert atmosphere to produce the LiFePO4—C composite. Because of the formation of a complex between metal cations and the resultant polymer, the resultant composites such-made are fine and their particle sizes are very uniform. This method is not efficient as additional care is required to prevent oxidization of the Fe(II) ions in the solution by air.
A common characteristic of the above processes is that the enhancement in the electronic conductivity of the LiFePO4—C composite is based on the formation of carbon on the surface of the cathode active material through a thermal decomposition of the organic polymer or small compound, whereas the thermal decomposition is greatly affected by the heating temperature and time. Low temperature and insufficient heating time cannot produce highly conductive carbon, while high temperature and long heating time not only induce growth of the product particle size but also cause reduction of the LiFePO4 by the resultant carbon. Therefore, electronic conductivity of carbon is limited by the trade-off between these two opposite effects.
On the other hand, Prosini et al (Electrochim. Acta, 46: 3517-3523, 2001) proposed an alternative method for the preparation of the LiFePO4—C composite by mixing 10 wt. % of carbon black, instead of organic polymer and compound, into the starting materials and then heating the resultant mixture at 800° C. for 16 h. U.S. Pat. No. 7,025,907 issued Apr. 11, 2006 to Kohzaki et. al. also used the similar approach, that is adding elemental carbon in one form or another mixed into the starting materials as the source of conducting carbon, but with special care taken in the heating step to not convert the iron present from ferrous (valence II) to ferric (valence III). The shortcoming of these methods is obvious. First, the physical contact between carbon and LiFePO4 particles is limited by the solid-state phase. Second, the content of carbon is high, which will induce reduction of the LiFePO4 active material due to the strong reductive property of carbon at high temperature. To solve these problems, U.S. Pat. Nos. 6,797,431, 6,811,924 and 6,814,764 propose a high speed hall-milling method to increase the physical contact of carbon and LiFePO4, while still employs as high as 10 wt. % of carbon black. This method adds a high-speed ball-milling step for from tens to over a hundred hours before the heating step. Such an approach is timely ineffective and cannot avoid the reduction of the LiFePO4 due to the presence of significant excess carbon.
U.S. Pat. No. 7,060,206 teaches the use of a significantly high amount of elemental carbon as both the reducing agent and carbon source. In another aspect, it teaches that the reducing carbon can be provided by a mixture of elemental carbon and organic precursor material. The organic precursor taught in this reference is a family of carbohydrate compounds, such as sucrose, which are incapable of complexing the transition metal ions and ineffective in reducing the particle size of the final products. Furthermore, the presence of the significantly high amount of carbon may reduce the product into the inactive Fe2P.
U.S. Pat. Nos. 6,720,112, 6,730,281 and 6,884,544 teach a carbothermal reduction method for the preparation of the LiFePO4—C composite active material, in which much excess amount of carbon is mixed with the mixture of lithium phosphate and iron (III) oxide in a stoichiometric molar ratio. The LiFePO4 cathode active material is formed through the thermal reduction of Fe(III) oxide by carbon. Because of the presence of significant excess amount of carbon, it is possible for the LiFePO4 to be reduced to FeP2, which consequently reduces the electrochemical characteristics of the composite cathode.